Fronthaul physical layer split in a cellular telecommunications network

ABSTRACT

This disclosure provides a method of operating a base station in a cellular telecommunications network, and a base station unit for implementing the method, the base station having a central base station unit and a distributed base station unit, wherein the central base station unit and distributed base station unit communicate over a fronthaul link having a first and second capacity configuration, and the cellular telecommunications network further includes a User Equipment (UE) consuming a service via the base station.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/EP2020/062656, filed May 7, 2020, which claims priority from EPPatent Application No. 19179353.8, filed Jun. 11, 2019, which is herebyfully incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a cellular telecommunications network.

BACKGROUND

A cellular telecommunications network may comprise a core network, aradio access network and a plurality of User Equipment (UE). Each UE mayaccess the core network (and any onward connection from the core networksuch as to the Internet) via the radio access network. A base station isan example of a radio access network node. The base station implementsseveral functions known as baseband processing. In an example, basebandprocessing includes the PHYsical-layer (PHY) layer functions, MediumAccess Control (MAC) layer functions, Radio Link Control (RLC) layerfunctions, Packet Data Convergence Protocol (PDCP) layer functions, andRadio Resource Control (RRC) layer functions.

In modern cellular telecommunications networks implementing aCentralized Radio Access Network (C-RAN), base station functionality maybe divided into several (typically two) components (either physically ifimplemented in hardware or logically if implemented in a softwaredefined network). These two components are commonly known as the CentralUnit (CU) and Distributed Unit (DU), although other terminology (such asBaseBand Unit (BBU) and Remote Radio Unit (RRU)) may be used. The CU andDU are connected by a fronthaul link, which may be a wireless or wired(typically optical fiber) connection. The DU implements at least a setof Radio Frequency (RF) functions (e.g. analog to digital and digital toanalog conversion) and optionally one or more baseband processingfunctions. The remainder of the baseband processing functions areimplemented in the CU. The split of functions between the CU and DU isknown as the “functional split”.

There are advantages and disadvantages in the choice of functional splitwhen the CU is connected to a plurality of DUs. That is, as morefunctions are implemented in the CU (so that fewer functions areimplemented in the DU) then the processing requirements for the DU arereduced and the CU may improve coordination across the plurality of DUs.However, such implementations generally have more stringent fronthaulrequirements, such as greater capacity and lower latency.

One possible functional split is known as the intra-PHY split, in whichthe RF and lower-PHY functions are implemented in the DU and allremaining functions are implemented in the CU. In this implementation,the CU and DUs communicate In-Phase and Quadrature (IQ) samples over thefronthaul connection.

SUMMARY

According to a first aspect of the disclosure, there is provided amethod of operating a base station in a cellular telecommunicationsnetwork, the base station having a central base station unit and adistributed base station unit, wherein the central base station unit anddistributed base station unit communicate over a fronthaul link having afirst and second capacity configuration, and the cellulartelecommunications network further includes a User Equipment (UE)consuming a service via the base station, the method comprising:communicating samples over the fronthaul link at a first capacityconfiguration and first bit width; determining that a reliabilitymeasure of the service is either less than or more than a reliabilitythreshold; and, (a) if the reliability measure of the service is lessthan the reliability threshold, responding by, (1) reconfiguring thesamples to use a second bit width, wherein the second bit width has morebits per sample than the first bit width, and (2) reconfiguring thefronthaul to use a second capacity configuration, wherein the secondcapacity configuration is greater than the first capacity configuration,and, (b) if the reliability measure of the service is more than thereliability threshold, responding by, (1) reconfiguring the samples touse a second bit width, wherein the second bit width has fewer bits persample than the first bit width, and (2) reconfiguring the fronthaul touse a second capacity configuration, wherein the second capacityconfiguration is less than the first capacity configuration;communicating samples over the fronthaul link at the second capacityconfiguration and the second bit width.

According to a second aspect of the disclosure, there is provided acomputer program product comprising instructions which, when the programis executed by a computer, cause the computer to carry out the method ofthe first aspect of the disclosure. The computer program may be storedon a computer-readable data carrier.

According to a third aspect of the disclosure, there is provided a basestation unit for a cellular telecommunications network, wherein thecellular telecommunications network includes a User Equipment (UE)consuming a service via the base station unit, the base station unitcomprising: a transceiver configured to communicate samples over afronthaul link with a distributed base station unit at a first capacityconfiguration and first bit width; and a processor configured todetermine that a reliability measure of the service is either less thanor more than a reliability threshold; and, (a) if the reliabilitymeasure of the service is less than the reliability threshold,responding by, (1) reconfiguring the samples to use a second bit width,wherein the second bit width has more bits per sample than the first bitwidth, and (2) reconfiguring the fronthaul to use a second capacityconfiguration, wherein the second capacity configuration is greater thanthe first capacity configuration, and, (b) if the reliability measure ofthe service is more than the reliability threshold, responding by, (1)reconfiguring the samples to use a second bit width, wherein the secondbit width has fewer bits per sample than the first bit width, and (2)reconfiguring the fronthaul to use a second capacity configuration,wherein the second capacity configuration is less than the firstcapacity configuration; wherein, following reconfiguration, thetransceiver is configured to communicate samples over the fronthaul linkat the second capacity configuration and the second bit width.

The fronthaul may be reconfigured to use the second capacityconfiguration by one or more of: switching to a different transmissionmode, switching to a different communications protocol, and switching toalternative or additional transmission hardware.

Reconfiguring the samples to use a second bit width may include:estimating a capacity requirement when communicating samples over thefronthaul using the second bit width, and comparing the estimatedcapacity requirement to the fronthaul's second capacity configuration.

The samples may be In-Phase and Quadrature (IQ) samples.

The central base station unit and UE may utilize a configurable errormitigation technique and the method may further comprise (or theprocessor may be further adapted to implement) reconfiguring the errormitigation technique.

Upon determining that the latency measure, or derivative thereof, of theservice is more than the threshold, then the second bit width may havemore bits per sample than the first bit width. Alternatively, upondetermining that the latency measure, or derivative thereof, of theservice is less than the threshold, then the second bit width may havefewer bits per sample than the first bit width.

Reconfiguring the error mitigation technique may include switchingbetween a first state, in which the error mitigation technique isactive, and a second state, in which the error mitigation technique isinactive.

BRIEF DESCRIPTION OF THE FIGURES

In order that the present disclosure may be better understood,embodiments thereof will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of an embodiment of a cellulartelecommunications network of the present disclosure.

FIG. 2 is a schematic diagram of a central base station unit of thenetwork of FIG. 1.

FIG. 3 is a schematic diagram of a distributed base station unit of thenetwork of FIG. 1.

FIG. 4 is a flow diagram of an embodiment of a method of the presentdisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of a cellular telecommunications network of thepresent disclosure will now be described with reference to FIGS. 1 to 3.FIG. 1 illustrates a Centralized Radio Access Network (C-RAN) 1including a Central Unit (CU) 10, a Distributed Unit (DU) 20 and a UserEquipment (UE) 30. The CU 10 and DU 20 are connected by a fronthaul link40.

The CU 10 is shown in more detail in FIG. 2. The CU 10 includes a firstcommunications interface 11 for connecting the CU 10 to the cellularcore network (via a backhaul link), a processor 13, memory 15, a secondcommunications interface 17 for connecting the CU 10 to the DU 20 (viathe fronthaul link 40), all connected via bus 19. In this embodiment,the first communications interface 11 is an optical fiber interface forconnecting the CU 10 to an optical fiber backhaul link, and the secondcommunications interface 17 is also an optical fiber interface forconnecting the CU 10 to an optical fiber fronthaul link. However, theskilled person will understand that other forms of backhaul andfronthaul links are possible, such as another form of wired connection(e.g. xDSL) or a form of wireless connection (e.g. operating accordingto a cellular telecommunications protocol).

The DU 20 is shown in more detail in FIG. 3. The DU 20 includes a firstcommunications interface 21 for connecting the DU 20 to the CU 10 viathe fronthaul link 40, a processor 23, memory 25, and a secondcommunications interface 27 for connecting the DU 20 to the UE 30 via anaccess link. In this embodiment, the second communications interface 27is an interface to an antenna for wireless communications with the UE30.

Turning back to FIG. 1, it is shown that the CU 10 and DU 20 performdifferent functions of the cellular telecommunications protocol. In thisembodiment, the DU 20 performs Radio Frequency (RF) functions (notshown) and lower PHYsical (PHY) layer functions, whilst the CU 10performs all higher layer functions including the higher PHY layerfunctions, Medium Access Control (MAC) functions, Radio Link Control(RLC) functions and Packet Data Convergence Protocol (PDCP) functions.This is known as an intra-PHY functional split. In this arrangement, theCU 10 and DU 20 communicate using frequency domain In-Phase andQuadrature (IQ) samples, transmitted over the fronthaul link 40.

xRAN Fronthaul Working Group Technical Specification “Control, User andSynchronization Plane Specification” v02.00 specifies (in Annex D)various IQ sample structures in which the number of bits used in each IQsample varies. The number of bits in an IQ sample is known as the “bitwidth”. These IQ sample bit widths range from 6 to 16 bits (inclusive).These different IQ sample bit widths are achievable using differentcompression levels. That is, the IQ sample may use a greater compressionlevel to achieve relatively fewer bits per sample instead of a lowercompression level to achieve relatively more bits per sample. Annex A ofthe above xRAN specification defines different compression techniquesthat may be used. The respective processors 13, 23 of the CU 10 and DU20 are able to implement one or more of these compression techniques toachieve any one of the available bit widths.

A performance of a receiver or transmitter may be defined by its ErrorVector Magnitude (EVM) parameter. The error vector is a vector in theI-Q plane between an ideal constellation point and a measured signal.The average amplitude of the error vector, normalized to peak signalamplitude, is the EVM. 3GPP Technical Specification 36.104 “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Base Station (BS) radiotransmission and reception” defines, in section 6.5.2, several EVMrequirements for E-UTRA carriers as:

TABLE 1 EVM Requirements for E-UTRA Carrier Modulation scheme RequiredEVM [%] QPSK 17.5% 16QAM 12.5% 64QAM   8% 256QAM  3.5% 1024QAM  2.5%

Table 1 illustrates that higher order modulation schemes requireimproved EVM performance (in which a lower EVM percentage correspondswith improved EVM performance). Furthermore, table 1 illustrates thatthere is a minimum EVM for each Modulation and Coding Scheme (MCS).

The following description relates to the transmitter EVM. However, it isnoted that the receiver EVM is also measurable and contributes to areduction in Signal-to-Interference-and-Noise-Ratio (SINR). That is, theeffect of bit width for uplink traffic can also be considered as aquantization effect, where reduced bit width can degrade the effectiveSignal-to-Interference-and-Noise-Ratio (SINR) of the received signal.

A first embodiment of a method of the present disclosure will now bedescribed with reference to FIGS. 1 and 4. In this embodiment, thecellular telecommunications network 1 is arranged as shown in FIG. 1 andthe CU 10 and DU 20 communicate using IQ samples having 6 bits persample (the smallest number of bits per sample according to the XRANspecification noted above) and thus use the highest compression level.In this starting scenario, the UE 30 is consuming a service from thecellular core network so that traffic for this service is transmittedbetween the UE 30 and cellular core network via the CU 10, DU 20 and thefronthaul link 40.

In this embodiment, the CU 10 and DU 20 utilize a fronthaul link 40 thathas an initial capacity that supports the current traffic for theservice. However, the CU 10 and DU 20 may vary this capacity byswitching from a low capacity configuration (for example, using 100 Mb/sEthernet) to a high capacity configuration (for example, using a 1 Gb/sEthernet). In this starting scenario, the fronthaul link 40 is using thelow capacity configuration and therefore only a proportion of itspotential capacity.

In S1 of this embodiment, the CU 10 monitors the access link to the UE30 to determine the error rate for the traffic for the service. Theerror rate may be a single measurement of the error rate at a particularlayer of the protocol stack, or a set of measurements of the error rateat a plurality of layers of the protocol stack.

In S3, the error rate between the CU 10 and UE 30 is compared to athreshold. This threshold is based upon the particular service beingconsumed by the UE 30, so that if the service requires, for example, aphysical layer block error rate of no more than 10%, then an error rateless than 10% will satisfy this threshold. The threshold may also bebased on other layers of the protocol, such as a MAC layer block errorrate of no more than 0.1% or and RLC layer block error rate of no morethan 0.0001%.

In an example scenario, the RF environment between the DU 20 and UE 30degrades such that the error rate between the DU 20 and UE 30 increasesabove the threshold. This degradation may be due to, for example, anobstacle moving between the DU 20 and UE 30. In the prior art, thisevent may be used as a trigger for the CU 10 to switch from its currentMCS to another. However, in this embodiment, the CU 10 does not switchMCS (because, for example, it is already using the most robust MCS) butinstead implements the following steps.

In S5, the CU 10 determines the maximum bit width (that is, a maximumnumber of bits per sample) possible. In this embodiment, thisdetermination is based on:

-   -   for the downlink, the maximum bit-width of the inverse Fast        Fourier Transform (iFFT) and the accuracy of both the power        amplifier and the digital to analog converter; and,    -   for the uplink, the maximum bit-width of the FFT, the accuracy        of both the analog to digital converter and receiver        sensitivity.

In this example, the CU 10 determines that the maximum bit width is 9bits per sample. Therefore, the fronthaul link 40 may use any one of 6,7, 8 or 9 bits per sample. In S7, the CU 10 then selects a bit widthfrom these candidate bit widths based on:

-   -   1. a comparison of the expected capacity requirements for the        fronthaul link 40 when using the candidate bit width to the        fronthaul link's capacity when operating under its high capacity        configuration; and    -   2. a comparison of an expected error rate when using the        candidate bit width to the error rate threshold for the service        (known from certification and/or calibration data). The expected        capacity requirements may be calculated as:

$R_{t} = {\sum\limits_{l = 1}^{L_{t}}{Q \cdot {PRB}_{t}^{l}}}$

Where R_(t) is the instantaneous user plane rate for Transmission TimeInterval (TTI) t; PRB_(t) ^(l) is the number of Physical Resource Blocks(PRBs) scheduled in layer l at time t; Q is the number of bits per PRB;and L_(t) is the number of co-scheduled UEs at time t.

The CU 10 then selects a candidate bit width which has an expectedcapacity requirement that is greater than the current capacity of thefronthaul link 40 (but within the potential capacity of the fronthaullink 40 when operating under its high capacity configuration) and has anexpected error rate that is below the error rate threshold used in S3(plus, optionally, a margin). In this example, the selected bit width is8 bits per sample.

In S9, the CU 10 and DU 20 cooperate to increase the capacity of thefronthaul link 40 by switching to the high capacity configuration. Inthis example, this is achieved by switching from a 100 Mb/s Ethernetconnection to a 1 Gb/s Ethernet connection. This step may also includean admission control function to remove resources being used by trafficof other services being transmitted over the fronthaul link 40 andreallocate them to the service.

Following reconfiguration to increase the capacity of the fronthaul link40, in S11, the CU 10 and DU 20 increase the bit width of the IQ samplesto 8 bits per sample. This is achieved by using a lower compressionlevel for the IQ samples. In S13, the CU 10 and DU 20 communicate usingIQ samples having the new bit width.

In conventional approaches, a CU 10 may respond to a worsening RFenvironment by improving the quality of the symbols used in thetransmission by modifying the MCS. As shown in Table 1 above, a changeto a more robust MCS (that is, one in which each symbol represents fewerbits) improves the EVM performance and thus improves the error rate.However, it may not always be appropriate to switch to a more robustMCS, such as when the access link between the DU 20 and the UE 30 isalready using the most robust scheme (either theoretically or based onthe service constraints). In this embodiment of the disclosure, the DU20 responds to the worsening RF environment by both increasing the bitwidth of the IQ sample and increasing the capacity of the fronthaul link40 to accommodate the corresponding increased fronthaul capacityrequirement. The effect of increasing the bit width of the IQ sample isto improve the EVM performance, so that the DU 20 may maintain itscurrent MCS whilst maintaining an acceptable error rate. Accordingly,the UE 30 may continue to consume the service in the degraded RFenvironment whilst maintaining an acceptable error rate. This embodimentof the invention is therefore particularly suitable for high reliabilityservices operating over a cellular telecommunications network, such asautonomous driving services. This embodiment is also particularlysuitable for control data communication, which should typically be morereliable than user data.

The process then loops back to S1 such that the DU 20 monitors the errorrate of the service and, in S3, compares the error rate to a threshold.In this second iteration of the method of this embodiment, the errorrate falls below the threshold. In conventional approaches, this wouldtypically be used as a trigger to switch the service to a less robustMCS (which would therefore use more bits per symbol and thereforeincrease capacity utilization of the link for the service). However, inthis second iteration, this event is used as a trigger to change the bitwidth and, possibly, reduce the capacity of the fronthaul link 40 byswitching to its low capacity configuration.

In S5 of this second iteration, the DU 20 determines the minimumpossible bit width, and, in S7, the DU 20 selects a reduced bit widthbased on:

-   -   1. a comparison of the expected capacity requirements of the        fronthaul link 40 when using the candidate bit width to the        fronthaul link's capacity when using its low capacity        configuration; and    -   2. a comparison of an expected error rate when using the        candidate bit width to the error rate threshold for the service.

In this example, the DU 20 selects a reduced bit width of 7 bits persample, which has both an expected error rate that satisfies the errorrate threshold for the service and has an expected capacity requirementthat would allow the fronthaul to switch to the low capacityconfiguration.

In S9, the DU 20 and CU 10 cooperate to switch to the low capacityconfiguration and, in S11, the CU 10 and DU 20 decrease the bit width ofthe IQ samples to 7 bits per sample. In S13, the CU 10 and DU 20communicate using IQ samples having the new bit width. In doing so, theadditional fronthaul capacity that has now become redundant (e.g. due tothe RF environment improving such that the increased bit width is nolonger necessary) is no longer being used. This reduces the energyrequirements of operating the CU 10 and DU 20, and releases thoseresources to be used by another entity.

In the above embodiments, the process of reconfiguring the bit width ofthe IQ samples and the capacity of the fronthaul link is triggered upondetection of a changing RF environment (or at least by a derivativemeasurement such as a changing error rate). However, the skilled personwill understand that this is not essential, and other triggers may beused. For example, the process may be triggered upon the UE 30requesting a new service, or an existing service being reclassified,whereby the requirements of the new or reclassified service exceed theresources of the fronthaul link 40.

In one implementation, it is possible to implement the above embodimentfor traffic for a particular service (e.g. the traffic for the new orreclassified service) instead of traffic for all services. Accordingly,the traffic for different services may be of different bit widths.

Furthermore, in the above embodiment, the fronthaul link 40 is anoptical fiber connection having a low capacity configuration and a highcapacity configuration. However, the skilled person will understand thatother forms of connection may be used having multiple capacityconfigurations. For example, the fronthaul link may be based on awireless connection and the capacity may be modified byenabling/disabling antennae or switching to a different communicationsprotocol. Furthermore, it is not essential that there is a singlefronthaul connection between the CU 10 and DU 20 having multiplecapacity configurations. Instead, the CU 10 and DU 20 may have amultipath fronthaul connection (e.g. using multiple optical fibersbetween the CU 10 and DU 20 or multiple connections between the CU 10and DU 20 of different forms) and the capacity may be modified by usinga different number of these connections.

In the second iteration of the above embodiment, the selected bit widthwas one that resulted in a reconfiguration of the capacity of thefronthaul link 40. However, the CU 10 may take a conservative approachin which the bit width is reduced in relatively small increments(compared to the increments used when increasing the bit width in thefirst iteration) which would not immediately permit a change in capacityof the fronthaul for several iterations. This provides some protectionfor a high reliability service in case the predicted error ratefollowing a reduction in bit width is too optimistic.

In the above embodiment, the CU estimated the capacity requirements of acandidate bit width and compared the estimated capacity requirement tothe potential capacity of the fronthaul link when using a particularcapacity configuration. However, the skilled person will understand thatthis comparison may be a function of the potential capacity ofparticular capacity configuration (e.g. 90% of the potential capacity),such that any subsequent reconfiguration does not use an unfairproportion of resources of the fronthaul link.

A second embodiment of a method of the present disclosure will now bedescribed. This embodiment utilizes the same CU 10, DU 20 and UE 30 asthe first embodiment, and the CU 10 and DU 20 again use an intra-PHYsplit such that communications between the CU 10 and DU 20 use IQsamples (initially at a bit width of 6 bits per sample). However, inthis second embodiment, communications between the CU 10 and UE 30 alsoutilize Automatic Repeat reQuest (ARQ), an example of an errormitigation technique which may be selectively applied by the respectiveprocessors of the CU 10 and UE 30 between a first state in which ARQ isapplied to communications and a second state in which ARQ is not appliedto communications. ARQ is a service specific error mitigation techniquethat is enabled in the RLC layer of the CU 10 and UE 30 for a servicethat requires a particular error rate. ARQ operates by the receivingnode requesting a retransmission from a sending node of a data packetthat is either not received within a particular time period or that isreceived but cannot be decoded. In response, the receiving node requestsa retransmission of that data packet by sending a feedback message tothe sending node. This delay (in sending the feedback message to thesending node and the sending node retransmitting the data packet)introduces a delay to the overall transmission which results inincreased latency. Accordingly, the ARQ technique is used to improvereliability, but degrades latency.

In this embodiment, the CU 10 monitors communications between the CU 10and UE 30 for the service to determine whether a latency metric (eitherthe latency or some derivative of latency, such as a count ofretransmissions) satisfies a threshold. If the latency surpasses thisthreshold, then the CU 10 reacts in the following manner. Firstly, theCU 10 and DU 20 cooperate to increase the bit width of IQ samplescommunicated over the fronthaul link. This is implemented in a similarmanner as described above for the first embodiment, in which the CU 10identifies a maximum bit width (based on the current capacity of thefronthaul link and the expected error rate of each bit width) toidentify a set of candidate bit widths (e.g. 8 bits per sample), selectsone of these candidate bit widths, and cooperates with the DU 20 toincrease the bit width of communications over the fronthaul link to thisselected bit width. Secondly, the CU 10 and UE 30 cooperate to disableARQ. This is implemented by the respective processors of the CU 10 andUE 30 switching from the first state to the second state.

Although the order of events (disabling ARQ and switching bit width) isnon-essential, it is noted that it is preferable to switch bit widthbefore disabling ARQ. This would increase the error rate performance inbetween the events, but would come at an increased resource utilizationpenalty. It would also be possible to disable ARQ before switching bitwidth (for example if resource utilization is already at or nearmaximum), but this may lead to an increased error rate in between theevents. It is possible to implement the two processes simultaneously,although inter-layer traffic must be considered (for example by applyingan offset).

The effect of increasing the bit width of communications over thefronthaul link is to reduce the overall error rate of communications forthe service. Accordingly, ARQ becomes redundant as an error mitigationtechnique, and so this technique is switched off and communications forthe service no longer suffer from latency degradation caused by ARQretransmissions. In effect, this second embodiment reacts to the triggerof latency increasing above a threshold by substituting a higher-layererror mitigation technique (ARQ) with a lower-layer error mitigationtechnique (increasing bit width), in which the lower-layer errormitigation technique offers improved latency performance over thehigher-layer error mitigation technique. Accordingly, this secondembodiment is particularly applicable to low latency (andhigh-reliability, low latency) services.

Following this reconfiguration, the CU 10, DU 20 and UE 30 continue tomonitor the latency of communications for the service and if the latencyimproves (e.g. satisfying a threshold) then these nodes may revert totheir original configuration by reducing the bit width of communicationsover the fronthaul link and switching ARQ on again. This reducesfronthaul utilization as the communications at the lower bit widthrequire fewer resources.

Although ARQ is used as an example, the skilled person will understandthat this is not essential. That is, there are many other errormitigation techniques that may be subject to the operations of thissecond embodiment. In other embodiments utilizing the same protocolstack, the error mitigation technique may be Hybrid ARQ (implemented inthe MAC layer), in which the trigger could be (for example) based on thecount of retransmissions for a successful transmission and thereconfiguration would be to reduce the limit on the number ofretransmissions for each data packet. In another embodiment, the errormitigation technique may be Transport Control Protocol (TCP)retransmission (implemented in the IP layer), in which the trigger wouldbe a count of retransmissions for a successful transmission and thereconfiguration would be to switch from TCP to the User DatagramProtocol (UDP). Other error mitigation techniques may apply for otherprotocol stacks.

Furthermore, the skilled person will understand that it is not essentialfor the trigger to be a direct measure of latency. That is, the triggermay be based on a derivative of latency, such as a count ofretransmissions or another measure linked to latency. That is, thedefinitions of latency and error rate are linked (as error rate is ameasure of the number of data packets that don't successfully arrive ina particular time period), so a threshold based on error rate could alsobe used as a trigger. Furthermore, the trigger may be based on theservice changing its latency requirements so that they are no longersatisfied by the current latency measurements.

The skilled person will understand that the methods of the first andsecond embodiments may be combined, so that the three reconfigurations(changing bit width, changing fronthaul capacity, and changing ahigher-layer error mitigation configuration) could all be used inparallel. This would be highly applicable to high-reliability, lowlatency services.

In the above embodiments, the base station implements one or morecompression techniques from the XRAN specification in order to vary thebit width from 6 bits per sample to 16 bits per sample. However, this isonly an example and other compression techniques may be used and otherbit widths may be used.

Furthermore, in the above embodiments, the method is implemented in theCU 10. However, the skilled person will understand that the method maybe implemented in the DU 20 either exclusively or in combination withthe CU 10 (with appropriate additional messages between the CU 10 and DU20 to communicate the necessary data). Furthermore, the skilled personwill understand that some or all of the steps of the method may beimplemented in a separate node to the base station, such as in an XRANcontroller with a connection to the CU 10 and/or DU 20.

The skilled person will understand that any combination of features ispossible within the scope of the invention, as claimed.

1. A method of operating a base station in a cellular telecommunicationsnetwork, the base station having a central base station unit and adistributed base station unit, wherein the central base station unit andthe distributed base station unit communicate over a fronthaul linkhaving a first capacity configuration and a second capacityconfiguration, and the cellular telecommunications network furtherincludes a User Equipment (UE) consuming a service via the base station,the method comprising: communicating samples over the fronthaul link atthe first capacity configuration and a first bit width; determining thata reliability measure of the service is either less than or more than areliability threshold; if the reliability measure of the service is lessthan the reliability threshold, responding by: reconfiguring the samplesto use a second bit width, wherein the second bit width has more bitsper sample than the first bit width, and reconfiguring the fronthaullink to use the second capacity configuration, wherein the secondcapacity configuration is greater than the first capacity configuration;if the reliability measure of the service is more than the reliabilitythreshold, responding by: reconfiguring the samples to use a third bitwidth, wherein the third bit width has fewer bits per sample than thefirst bit width, and reconfiguring the fronthaul link to use a secondcapacity configuration, wherein the second capacity configuration isless than the first capacity configuration; and communicating samplesover the fronthaul link at the second capacity configuration and thesecond bit width or the third bit width.
 2. The method as claimed inclaim 1, wherein the fronthaul link is reconfigured to use the secondcapacity configuration by one or more of: switching to a differenttransmission mode, switching to a different communications protocol, orswitching to alternative transmission hardware or additionaltransmission hardware.
 3. The method as claimed in claim 1, whereinreconfiguring the samples to use the second bit width or the third bitwidth, respectively, includes: estimating a capacity requirement whencommunicating samples over the fronthaul link using the second bit widthor the third bit width, respectively, and comparing the estimatedcapacity requirement to the second capacity configuration of thefronthaul link.
 4. The method as claimed in claim 1, wherein the samplesare In-Phase and Quadrature (IQ) samples.
 5. The method as claimed inclaim 1, wherein communications between the central base station unitand the UE utilize a configurable error mitigation technique, and themethod further comprises reconfiguring the error mitigation technique.6. A computer program product comprising instructions which, when theprogram is executed by a computer, cause the computer to carry out themethod of claim
 1. 7. A computer-readable data carrier having storedthereon the computer program of claim
 6. 8. A base station unit for acellular telecommunications network, wherein the cellulartelecommunications network includes a User Equipment (UE) consuming aservice via the base station unit, the base station unit comprising: atransceiver configured to communicate samples over a fronthaul link witha distributed base station unit at a first capacity configuration and afirst bit width; and a processor configured to determine that areliability measure of the service is either less than or more than areliability threshold; if the reliability measure of the service is lessthan the reliability threshold, responding by: reconfiguring the samplesto use a second bit width, wherein the second bit width has more bitsper sample than the first bit width, and reconfiguring the fronthaullink to use a second capacity configuration, wherein the second capacityconfiguration is greater than the first capacity configuration; if thereliability measure of the service is more than the reliabilitythreshold, responding by: reconfiguring the samples to use a third bitwidth, wherein the third bit width has fewer bits per sample than thefirst bit width, and reconfiguring the fronthaul link to use a secondcapacity configuration, wherein the second capacity configuration isless than the first capacity configuration; and wherein, followingreconfiguration, the transceiver is configured to communicate samplesover the fronthaul link at the second capacity configuration and thesecond bit width or the third bit width.
 9. The base station unit asclaimed in claim 8, wherein the transceiver is reconfigured to use thesecond capacity configuration by one or more of: switching to adifferent transmission mode, switching to a different communicationsprotocol, or switching to alternative transmission hardware oradditional transmission hardware.
 10. The base station unit as claimedin claim 8, wherein reconfiguring the samples to use second bit width orthe third bit width, respectively, includes: estimating a capacityrequirement when communicating samples over the fronthaul link using thesecond bit width or the third bit width, respectively, and comparing theestimated capacity requirement to the second capacity configuration ofthe fronthaul link.
 11. A base station unit as claimed in claim 8,wherein the samples are In-Phase and Quadrature (IQ) samples.
 12. Thebase station unit as claimed in claim 8, wherein the base station unitis a central base station unit configured to communicate with the UEusing a configurable error mitigation technique, wherein the processoris further configured to reconfigure the error mitigation technique inresponse to the reliability measure satisfying the threshold.